1. Field of the Invention
This invention relates to composite materials for dental restorations. In particular, this invention relates to fiber reinforced prosthodontic frameworks comprising a fiber reinforced composite framework and at least one or more filled composite veneers.
2. Brief Discussion of the Art
Fiber-reinforced composites have found increasing use in the field of materials for dental restorations, and are described, for example, in U.S. Pat. Nos. 4,717,341 and 4,894,012 to Goldberg et al. both of which are hereby incorporated by reference in their entirety. Fiber-reinforced composites generally comprise at least two components, a polymeric matrix and fibers embedded within the matrix. The polymeric matrix may be selected from those known for use in composite dental materials, for example polyamides, polyesters, polyolefins, polyimides, polyarylates, polyurethanes, vinyl esters or epoxy-based materials. The fibers used to reinforce composite material may comprises glass, carbon, or polymer fibers such as polyaramide and polyethylene, as well as other natural and synthetic fibers.
Fiber reinforced composite material provides several advantages, most notably increased strength and stiffness. As described in U.S. Pat. Nos. 4,717,341 and 4,894,012 to Goldberg et al., such materials accordingly are used as structural components in a variety of dental appliances, taking the form of bars, wires, beams, posts, clasps, and laminates for use in traditional bridges, crowns, artificial teeth, dentures, veneers, and the like. They have also been used in connection with orthodontic retainers, bridges, space maintainers, splints, and the like. In these applications, the fibers preferably take the form of long, continuous filaments, although the filaments may be shorter than 5 millimeters. Where the composites take the form of elongated wires, the fibers are at least partially aligned and oriented along the longitudinal dimensions of the wire. However, depending on the end use of the composite material, the fibers may also be otherwise oriented, including being normal or perpendicular to that dimension.
Fiber reinforced composites are particularly useful as structural components in dental bridges. In the dental arts, a bridge is a device for the restoration and replacement of one or more natural teeth, replacing at least one missing tooth and supported on either side by the remaining teeth. A bridge generally comprises a pontic for replacement of the missing tooth, and a connector on either side of the pontic which connects the pontic to a retaining member such as a crown formed on an abutment tooth adjacent the pontic. By their nature, bridges must be aesthetic, as well as strong, in order to withstand forces generated by chewing and to maintain the positions of the abutting teeth.
The combination of a fiber reinforced framework and particulate filled composite veneer offers good strength and excellent aesthetics. These systems result in a decrease in the antagonistic wear of opposite teeth compared to the use of metal-porcelain bridges. These systems also provide higher impact energy, and are free of leaching of metal ions. The tensile strength and elastic modulus of uniaxially oriented continuous glass fiber reinforced BIS-GMA are competitive with those of stainless steel and some titanium alloys. Such bridges are described for example in co-assigned U.S. Provisional Patent Application No. 60/055,590, filed Aug. 12, 1997, now U.S. Pat. No. 6,039,569, and exemplified by the FIBREKOR® system for making dental bridges commercially available from Jeneric/Pentron Inc., Wallingford, Conn.
An important consideration in constructing a single or multi exerted on the bridge. Dental restorations must be able to withstand the normal mastication forces and stresses that exist within an oral environment, which have been described, for example, in “Restorative Dental Materials”, 4th ed., edited by F. A. Peyton and R. G. Craig, pp. 121-133 (1971). Different stresses are observed during mastication of different types of food, which can be experimentally measured by placing, for example, a strain gauge in inlays on the tooth. Stresses differ depending not only on the type of food, but also on the individual. For example, stress values may range from 570 to 2300 lb./inch2 or from 950 to 2400 lb./inch2 for a single thrust. The physical properties of dental restorations must be adequate to withstand the stresses applied by the repetitive forces of mastication. If an applied force exceeds that which the dental restoration can withstand, then fracture in the dental restoration material results. Therefore, the dental restoration must be constructed so that loads on the restoration are lower than the maximum load-bearing capability of the restoration.
An important parameter in dental bridges in particular is the flexural strength of such bridges. In a multi-unit dental bridge there is at least one pontic not supported on its gingival surface. The only supports are the two connecting areas with the adjacent abutments. Hence if a load is applied normal to any pontic surface, the bridge tends to deflect, resulting compressive stress/strain on the surface on which load is applied and tensile stress/strain on the opposite surface. This is common for any simply-supported specimen exposed to flexural testing. Because of the geometric complexity of the dental bridge and the multidirectional loads generated in different locations during chewing and mastication, the magnitudes of stress/ strain vary in different locations.
Consequently, while well-suited for their intended purposes, the design of many currently manufactured dental bridges suffers from a framework material having a flexural modulus higher than that of the particulate-filled veneer material; and/or the particulate-filled veneer material having a strain to failure value lower than that of the fiber-reinforced framework material. There is a need to make the particulate filled composites compatible with the fiber reinforced composite structural frameworks in the strain to failure value to provide high strength to the dental restoration. It is desirable to increase strength of dental restorations without complicating or increasing the number of steps used in the fabrication of dental restorations. Such improvements would result in dental bridges which can better withstand the forces and strains that accompany the chewing of food and other activities, and which provide maximum performance for their intended use.